This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Fuel cell technology is considered to be a potential next generation energy solution for powering stationary systems, portable electronic devices, and vehicles. With hydrogen as a fuel, fuel cell technology is environmentally friendly, as it generates electricity from the hydrogen oxidation reaction at the anode and the oxygen reduction reaction (ORR) at the cathode, producing water as the only by-product. Fuel cells are rapidly becoming an important component in the energy industry. Currently, however, costly platinum catalyst is typically a required component of the fuel cell. Replacing expensive platinum as catalyst is a significant challenge for large-scale application of fuel-cell technology. In addition to high cost, substantially pure platinum is neither the most active, nor the most stable catalyst for fuel cell reactions. Specifically, the instability of platinum at the cathode side represents one of the major limitations for commercialization of fuel cell technology.
To make fuel cells commercially competitive at reasonable cost, the amount of platinum used in the fuel cell should be reduced by a factor of four to five. Further, catalyst stability should be improved for longer term operation of the fuel cell. Platinum in the form of nanoparticles dispersed on a high surface area carbon matrix is considered to be the catalyst of choice for the hydrogen oxidation and ORR reactions. Although platinum based alloys embedded in high surface area carbon substrates have been developed that improve catalyst performance by a factor of two, these efforts have not increased catalyst stability, and have not reduced the total loading of platinum in fuel cells to economically feasible levels.
Several efforts have focused on improving the activity of platinum catalysts by alloying platinum with transition metals such as Fe, Co or Ni. For example, extended surfaces of these alloys has achieved enhanced activity that originates from modified electronic structures of platinum, which alters the adsorption of spectator species from the electrolyte and the binding energies of key reaction intermediates, and thus improves the reaction kinetics. Apart from the quest for more active systems, less focus has been placed on the stability of the platinum catalyst. Although, platinum is generally chemically inert, it becomes unstable when exposed to a hostile electrochemical environment such as the ORR. Under such conductions, platinum surface atoms dissolve and migrate, resulting in aggregation of nanoparticles and losses of surface area, activity and power density.
It has been reported that platinum surface sites with low coordination numbers such as step edges, corners, kinks, and adatoms are more vulnerable to dissolution than the atoms that are part of long-range ordered (111) or (100) facets. For example, scanning tunneling microscopy (STM) studies combined with electrochemical and infrared characterizations from platinum single crystal surfaces covered with adsorbed CO has confirmed dissolution. It has also been found that the adsorption of surface oxides occurs on low-coordinated platinum sites. Once formed, the rather strong Pt-oxide interaction induces substantial morphological changes of the topmost surface atoms triggering decay in fuel cell performance.
The instability of platinum at the cathode side represents a major limitation for commercialization of fuel cell technology. As such, there is a need to develop advanced catalytically active materials with not only high activity, but also superior durability and less costly than Pt alone.